The Cincinnatian Series (Upper Ordovician) is well exposed in its type region of the Indiana-Ohio-Kentucky area in North America. The highly fossiliferous rocks contain abundant and well-preserved scolecodonts indicating that rich jawed polychaete annelid faunas inhabited the shallow subtropical Ordovician sea. An extensive collection of well over 50 000 scolecodonts from a c. 10 Ma interval spanning the Edenian (the Amorphognathus superbus Conodont Zone and the Climacograptus spiniferus Graptolite Zone) to the latest Richmondian (the A. ordovicicus Conodont Zone and the Dicellograptus complanatus Graptolite Zone), is currently under study. Preliminary results reveal that some 40 to 50 jawed polychaete species, belonging to a dozen families, occur in the succession. The most common and/or characteristic polychaete genera are Oenonites Hinde, Kettnerites Zebera, Ramphoprion Kielan-Jaworowska, Protarabellites Stauffer, “Arabellites” Hinde, Kalloprion Kielan-Jaworowska, Leptoprion Kielan-Jaworowska, and Hadoprion Eriksson & Bergman. One family, the Polychaetaspidae, markedly dominates and generally represents more than 60% of the jawed polychaete faunas. At certain horizons, particularly in the lower part of the Richmondian Waynesville Formation, polychaetaspids comprise more than 95% of the jawed polychaete fauna. Although being highly abundant, the polychaetaspids show low species diversity and only one or two species normally occur in each stratigraphic interval. In general, the taxonomic diversity increases from the shale-dominated Edenian Kope Formation to the limestone-dominated Richmondian Whitewater Formation and, hence, increases with decreasing water depth. The Cincinnatian polychaete faunas can be subdivided into at least four more or less distinct faunal associations of potential biostratigraphic utility. Moreover, congeneric and conspecific taxa occur in coeval strata of Baltica and indicate that several of these benthic organisms had an intercontinental distribution during Late Ordovician time. There are, however, some differences in faunal composition between Baltica and Laurentia. Placognath forms and members of the Polychaeturidae are considerably more common in Baltica than in Laurentia whereas hadoprionids have not been identified in Ordovician rocks of Baltica.